It has been found to be very useful to have enzymes in dishwashing detergent compositions because enzymes are very effective in removing food soils from the surface of glasses, dishes, pots, pans and eating utensils. The enzymes attack these materials while other components of the detergent will effect other aspects of the cleaning action. However, in order for the enzymes to be highly effective, the composition must be chemically stable, and it must maintain an effective activity at the operating temperature of the automatic dishwasher. Chemical stability is the property whereby the detergent composition containing enzymes does not undergo any significant degradation during storage. This is also known as shelf life. Activity is the property of maintaining enzyme activity during usage. From the time that a detergent is packaged until it is used by the customer, it must remain stable. Furthermore, during customer usage of the dishwashing detergent, it must retain its activity. Unless the enzymes in the detergent are maintained in a suitable environment, the enzymes will suffer a degradation during storage which will result in a product that will have a decreased initial activity. When enzymes are a part of the detergent composition, it has been found that the initial free water content of the composition should be as low a level as possible, and this low water content must be maintained during storage, since water will activate the enzymes. This activation will cause a decrease in the initial activity of the detergent composition.
After the detergent container is opened, the detergent will be exposed to the environment which contains moisture. During each instance that the detergent is exposed to the environment it could possibly absorb some moisture. This absorption occurs by components of the detergent composition absorbing moisture, when in contact with the atmosphere. This effect is increased as the container is emptied since there will be a greater volume of air in contact with the detergent, and thus more available moisture to be absorbed by the detergent composition. This will usually accelerate the decrease in the activity of the detergent composition. The most efficient way to prevent a significant decrease in this activity is to start with an initial high activity of enzyme and to use components in the dishwashing composition which have a low hygroscopicity and a low alkalinity which will minimize any losses in activity as the detergent is being stored or used.
The stability of enzymes in a nonaqueous liquid detergent can be improved by using an alkali metal silicate. In addition, the individual components of the detergent composition should each have an initial free water content (unbounded water at 100.degree. C.) of less than about 10 percent by weight, more preferably less than about 9 percent by weight, and most preferably less than 8 percent by weight. During manufacture the detergent composition may take-up moisture from the atmosphere. As a result, the moisture content of the detergent composition as it is being packaged may be greater than about 1 percent by weight, preferably less than about 4 percent by weight and most preferably less than about 3 percent by weight.
Nonaqueous liquid dishwasher detergent compositions which contain enzymes can be made more stable and to have a high activity, if the initial free water content of the detergent composition less than about 6 percent by weight, more preferably less than about 4 percent by weight and most preferably less than about 3 percent by weight. A key aspect is to keep the water (non-chemically bonded water) in the detergent composition at a minimum. It is critical that water not be added to the composition. Absorbed and absorbed water are two types of water and comprise the usual free water bound in the detergent composition. Free water will have the affect of deactivating the enzymes. Furthermore, the pH of 1.0 weight % of an aqueous solution of a liquid detergent composition must be less than about 11.0 more preferably less than about 10.8, and most preferably less than about 10.5. This low alkalinity of the dishwashing detergent will also increase the stability of the detergent composition which contains a mixture of enzymes, thereby providing a higher initial activity of the mixture of the enzymes and the maintenance of this initial high activity.
The free water content of the dishwashing detergent compositions of the instant invention can be controlled to a large extent by using components that have a low initial water content and a low hygroscopicity. The individual components of the instant composition should have a water content of less than about 10 percent by weight, more preferably less than about 9 percent by weight, and most preferably less than about 8 percent by weight. In addition, the organic components of the dishwashing detergent composition should have low hydroxyl group content to decrease the hydrogen bonding absorption of water. In place of the carrier such as ethylene glycols or glycerols, relatively low hydroxyl content-anhydrous organics such as alcohol ethers and polyalkylene glycols can be used. In place of polyacid suspending agents normally used in liquid automatic dishwashing detergent compositions such as polyacrylic acid or salts of polyacrylic acids, there should be used polyacid/acid anhydride copolymers such as polyacrylic acid/acid anhydride copolymers. Maleic anhydride is a suitable acid anhydride. The net result is a decreased hydroxyl group content which translates to a decreased hygroscopicity of the detergent composition which helps maintain the stability and the activity.